1. Field of the Invention
The invention relates to an electronic ballast for a gas discharge lamp, and more particularly to such a ballast which enables accurate control of the lamp intensity by an externally supplied dimming signal for adjusting the power supplied to the lamp even at very low illumination levels (e.g. 1 or 2% of maximum intensity) and even when the ballast is coupled to the lamp by remote wiring having significant stray capacitance.
2. Description of the Related Art
U.S. Pat. No. 5,742,134, issued Apr. 21, 1998 to the present Applicants and assigned to the present Assignee, Philips Electronics N.A., discloses an electronic ballast comprising a half-bridge inverter which includes a pair of serially connected MOSFET switches for generating a high frequency square wave in a resonant output circuit in which the lamp is connected. The inverter is driven by a drive control circuit principally consisting of an integrated circuit (IC) having pins corresponding to various operating parameters of the ballast, such as lamp current, voltage and power, as well as a pin for receiving an external dimming control signal. A feedback loop in the IC controls the lamp intensity by varying the switching frequency of the inverter, a change in frequency in the vicinity of resonance of the inverter output circuit causing a substantial change in lamp current and voltage and consequently in the power supplied to the lamp. A signal which is used as a measure of lamp power is obtained as the product of measured average lamp current and measured average lamp voltage, which power signal is used to derive an error signal for adjusting the lamp intensity to a level signified by an externally supplied dimming signal. Linear control of lamp intensity is thereby provided over a range down to as low as 1 or 2% of full intensity. Such patent is incorporated herein by reference, and constitutes a part hereof as fully as if set forth herein.
A problem is encountered with such a ballast when it must be located at some distance from the lamp, so that remote wiring is necessary which introduces significant parasitic capacitance there-between. As a result of such capacitance the lamp current and voltage as detected at the ballast can have a significant phase difference. Consequently, the actual lamp power is no longer simply the product of average (i.e., DC) lamp current and voltage but rather is given by an integration of the product of actual lamp voltage and current over each periodic cycle thereof. At low intensity levels the lamp current may be of the same order of magnitude as the parasitic capacitive current, and so the current and power as measured at the ballast can become altogether erroneous as a measure of actual lamp current and power. That will result in erroneous control of lamp intensity, and can also cause difficulty in lamp start-up because the parasitic current will be interpreted by the ballast as lamp current and consequently as an indication that ignition has occurred and therefore that the lamp voltage can be reduced from the required start-up level.